Abstract

We have studied the propagation of quasiparticles in a superconducting film which is penetrated by a magnetic field. We inject quasiparticles into an Al film at 75 mK using a normal–insulator–superconductor tunnel junction and measure the flux of quasiparticles which diffuse into an adjoining thermal detector. A magnetic field applied perpendicular to the plane of the film produces regions of reduced energy gap which trap quasiparticles. We find a significant reduction in the quasiparticle flux for fields as small as 2 G and explain our measurements with a model that incorporates inelastic scattering rates in Al and the characteristics of field penetration.